Reducing power consumption is an important issue in many modern personal computers. This issue is particularly important for battery-powered lap-top or notebook style portable computers. Other computers with a limited supply of power also need to be designed with a concern for power conservation. Computers with a limited supply of power are often those designed to be portable. In order to achieve the portability of these computer systems, the power consumed by the system components must be minimized. Reduced power consumption prolongs the battery life and thus increases the feasibility of the computer as a portable system. Clearly, the simplest way to conserve power is to shut the system down entirely; however, shutdown destroys the contents of memory and forces the user to restart and re-boot the computer from scratch. A better method is to provide a reduced power mode which supplies a minimal level of power to vital subsystems thereby saving the processing state of the computer.
Many of these portable computers include a memory subsystem for storing data and instructions for the processor and a video subsystem for storing and displaying a video image on a display screen. The memory subsystem and a video memory in the video subsystem may be configured using dynamic random access memory (DRAM) devices for storing information. DRAMs require a cyclic refresh signal supplied by the computer system in order to prevent the information stored in DRAM from being destroyed. It is advantageous to implement a reduced power mode wherein the operation of the computer is suspended, but the memory subsystem and video memory is saved. In this way, the computer user may reactivate the system after a suspended period and obtain the same system state and display as present prior to the suspension.
Several systems and methods exist in the prior art for providing a reduced power mode in a computer with a limited source of power. One such system saves the contents of system memory and video memory on a hard disk or a non-volatile memory prior to removing power from the system. This method has several disadvantages. First, the computer system must be configured with a hard disk or non-volatile memory resource. A particular computer may not be so configured. Also, a hard disk or non-volatile memory is an expensive component which raises the overall cost of the system. Secondly, the disk access necessary for saving and restoring memory is a relatively slow operation. There may be a delay in response to the entry and return from the suspend mode. Thirdly, if the suspend mode is entered as a result of a low battery condition, there may not be enough power to access the disk. Disk access is a relatively power intensive operation.
Other systems providing a reduced power mode in a computer with a limited source of power implement such a mode by using static RAM (SRAM) or pseudo static RAM (PSRAM) for system and video memory. SRAM does not require a cyclic refresh signal as required by DRAM. SRAM can therefore operate at a low power consumption rate during a suspend mode without losing data. PSRAM must be put into a standby mode in order to operate the device without refresh. Unfortunately, SRAMs are substantially more expensive than DRAMs. Thus, the overall system cost increases and the competitiveness of the computer system in the marketplace decreases.
Still other systems implement a video memory save operation where the video memory contents are saved in a system memory area. This technique, however, does not reduce the power consumption in the computer system. If system memory is DRAM, the system memory must still be refreshed during a suspend mode. If system memory is SRAM or some form of non-volatile memory, the system cost increases as mentioned above. In addition, such systems may require additional system memory in order to have enough space to hold the contents of video memory during a power suspend mode. Additional software may also be required to control the allocation and maintenance of system memory used to hold video memory during suspend mode. This additional software may lead to a specially configured BIOS thereby reducing the general applicability of the BIOS supplied with a basic system.
A better computer system design is needed for implementing a more efficient and less expensive power suspend mode.